Method and device for electrostatic spraying of material

ABSTRACT

A method and apparatus for electrostatically charging material to be sprayed onto a surface, by pressing the material to be sprayed between two electrodes of a condenser positioned in a spaced-apart relationship and insulated from each other by a dielectric. One of the electrodes is connected to a high-voltage, low-amperage power supply and the other electrode is grounded to create a uniform electrical field through which the material to be sprayed is passed for achieving a strong, uniform electrostatic charging of all the material to be sprayed while requiring less voltage and current for operation than the prior art devices and methods.

United States Patent [191 Benedek et al.

[451 May 29, 11973 [54] METHOD AND DEVICE FOR ELECTROSTATIC SPRAYING OF MATERIAL [22] Filed: July 23, 1971 [21] Appl. No.2 165,606

[30] Foreign Application Priority Data July 31, 1970 Hungary ..HA-893 [52] US. Cl. ..239/l5, 117/93.4, 118/621 51 rm. Cl. ..B05b 5/02 [58] Field of Search ..239/3, 15; l17/93.4; 118/621, 629

[56] 1 References Cited 2,858,798 11/1958 Sedlacsik ..239ll5 X 3,352,285 11/1967 Buhmann ....l l7/93.4 R X 3,587,967 6/1971 Badger ..239/15 3,604,980 9/1971 Robertson ..239/l5 X Primary Ezgarni r er -Robert S Ward, Jr. Attorney Richard C. Sughrue, Gideon Franklin Rothwell, John H. Mion et al.

[57] ABSTRACT A method and apparatus for electrostatically charging material to be sprayed onto a surface, by pressing the material to be sprayed between two electrodes of a condenser positioned in a spaced-apart relationship and insulated from each other by a dielectric. One of the electrodes is connected to a high-voltage, low-amperage power supply and the other electrode is grounded to create a uniform electrical field through which the material to be sprayed is passed for achieving a strong, uniform electrostatic charging of all the material to be sprayed while requiring less voltage and current for operation than the prior art devices and methods.

6 Claims, 3 Drawing Figures 1. METHOD AND DEVICE FOR ELECTROSTATIC SPRAYING OF MATERIAL This invention relates to a method and a device for the electrostatic spraying of dyestuffs, powder, fibrous substances and the like, in which the material to be sprayed is charged electrostatically and is then applied to an object which has the opposite electrical charge. Such electrostatic spraying methods and devices are already known for coating objects with corrosionprotecting or decorative material or for the application of a dye layer. In the prior art, an electrode is used to charge the material to be sprayed, with this electrode having a metallic or a semiconductor connection with a power source having a voltage of 50-150 kV. This electrode charges the material to be sprayed either by direct contact or through ionized air layers.

In the prior art, the size of the force field which develops along the electrical conductors or in their immediate vicinity changes in inverse proportion to the dimension of the roundoff radii of the surfaces to which the voltage is applied, so that very strong force fields develop along cutting edges or points while the potential remains the same. With the help of these force fields the resultant charge can be increased considerably. It is therefore already known that a good electrostatic charge of a material to be sprayed can be a'chievedby using electrodes which are designed with metal points or metal cutting edges, so that when the object to be coated, which acts as the counterelectrode, is relatively close to the front of the electrode designed in the form of a metal tip or metal cutting edge, a strong force field can develop between the electrodes. Spraying devices have therefore already been provided in which electrodes are built into the spraying head in the form of ionizing needles or with sharp edges having a bell-shape or like construction.

For preventing accidents during the operation of the spraying apparatus, it is absolutely necessary that the high voltage source have a low short circuit current be about 200 microA. However, even with this low current value, it is extremely important that the electrical charge contained in the spraying device is not discharged undarnped. A choke resistance of several Megohoms has therefore been provided.

Electrostatic spraying devices are also known in which the transfer of the electrical charge to the material to be sprayed is accomplished by means of a metal electrode arranged in the interior of the spraying head.

If this electrode in the spraying head is placed as far toward the interior as possible and if it is provided with adequate insulation, then these known devices are indeed completely spark proof. However, they do entail the disadvantage that the charge of the material to be sprayed, which is achieved through them, as a rule, is not sufficiently strong to accomplish uniform coating. The problem is that in this type of equipment, a sufficiently strong charge is essentially produced only in the core and not in the outer sections of the dispersion cone.

The purpose of this invention, therefore, is to present a method and device for the electrostatic spraying of dyestuffs, powdery materials, fibrous materials or the like, of the type mentioned above, with which the best possible uniform charge of the material to be sprayed and thus the most uniformly possible homogeneous v value. The value of this current has been determined to coating can be achieved with the smallest possible required electrical voltage. Here, a stronger charge of the material to be sprayed is achieved, wherever possible, with the use of a lower high voltage than has been used so far to date. This is accomplished according to the present invention in a method of the kind described previously, in the following manner: the material to be sprayed is conducted, prior to its being sprayed, between the two electrodes of a condenser which are positioned opposite each other a short distance apart, the electrodes being insulated by a dielectric, and charged electrically with a different polarity by means of high voltage. The material to be sprayed is then electrically charged in the electrical field of this condenser.

According to the prior art methods, the force field for charging the material to be sprayed is constituted by the electrode in the sprinkling head and by the object to be sprayed which serves as a counter-electrode. These electrodes, as a rule, are spaced at an interval of about 15-50 cm from each other. In the method according to the invention, on the other hand, a force field is used for charging the material to be sprinkled. This force field is constituted between two electrodes of an electrical condenser which are positioned opposite each other at a small interval of a few millimeters or tenths of a millimeter, with the material to be charged being conducted between these two electrodes prior to reaching the spraying head. The force field between these two closely adjoining electrodes of a condenser is, with an applied voltage which is smaller than in the hitherto prior art methods, considerably greater than the force field which is developed in the known prior art methods between the electrode in the spraying head and the object to be sprayed, which acts as the counter-electrode, so that the charging of the materials which are to be sprayed, which are conducted between the two electrodes of the condenser in the method of the present invention, is considerably stronger and more uniform than in the past. This charging is independent of the distance between the spraying head and the object to be sprayed and is also more uniform than in the past. This charging is independent of the distance between the spraying head and the object to be sprinkled and is also independent of whether the object to be sprayed has a level or uneven surface, for example, with protruding points or tips, which, in the known method, incline toward the increased accumulation of the sprayed material.

In the known prior art method, involving the use of an electrode arranged in the spraying head in the form of a tip or cutting edge, an average field intensity of 3-5kV/cm is developed in the case of an applied voltage of kV and an object to be sprayed having an almost level surface and spaced at a distance of 20 cm from the electrode in the spraying head. By means of the corresponding design of the electrode, for example, through further sharpening or pointing, it has been possible to achieve a deformation of this force field and, consequently, along with a light phenomenon and a corona discharge, it was possible to achieve a field intensity of about 30-50 kV/cm. In the method according to the invention, on the other hand, the field intensity of 30-50 kV/cm, which is completely sufiicient for spraying, is assured, even with an applied voltage of only 15-20 kV, by means of the two electrodes of the condenser which are spaced-apart at a small interval of a few millimeters or tenths of a millimeter from each other, these electrodes forming the condenser through which the material to be sprayed passes prior to spraying. Furthermore, this method according to the present invention guarantees the extraordinary uniform charging of the material to be sprayed by means of this force field.

This invention also relates to a spraying device for the effective implementation of the method according to the present invention, which uses a spraying head and a duct for the supply of the spraying material to the spraying head, and having an electrode which is connected to a high-voltage source. A charge is supplied to the electrode from this high-voltage source, that differs with respect to the object to be sprayed. This spraying device is characterized, according to the present invention, by the fact that the electrode, which is differently charged with respect to the object to be sprayed, is positioned opposite a counter-electrode at a small interval therebetween, and, along with this counter-electrode, forms a condenser through which the material to be sprayed is conducted. This condenser is arranged in front of the spraying head with the supply duct for the material. As stated hereinabove, the interval or spacing between the two electrodes of the condenser inside the supply duct to the sprinkler head may be a few millimeters or tenths of a millimeter. One of the two electrodes is practicalably coated with a dielectric which faces toward the opposite electrode. The material to be sprayed is conducted between the coated electrode and the uncoated electrode of the condenser, whereby, in the case of an applied voltage of 20 kV and an electrode interval of 2 mm, a force field of 100-200 kV/cm is developed. This force field is considerably greater than the force fields which have been obtained with the hitherto known methods and devices.

The electrode, which is connected to the highvoltage power source and which is diflerently charged by that source with respect to the object to be sprayed, is spaced further from the exit end of the supply duct on the spray head of the device than the other electrode of the condenser which is positioned opposite the charged electrode. Both electrodes of the condenser are arranged in the interior of the sprinkler device and are effectively insulated with respect to the outside surfaces of the equipment, more specifically, through a reliable operating safety insulating layer and through a corresponding insulation interval, in order to make the equipment completely sparkproof and accident proof. As a result of the small electrode spacing, the equipment can be powered with a considerably lower voltage than in the hitherto known equipment units, which further reduces the danger of accident. In practice this means that, compared to the hitherto used voltage, a voltage of only -30 kV will sufl'ice for the adequate charging of the material to be sprayed, in the method and equipment according to this invention.

According to one preferred version, the two electrodes of the condenser can be made up of pipes which are placed concentrically inside each other, whereby the supply duct for the material to be sprayed is located between these two pipes which are separated by a dielectric. This duct can have a cylindrical or spiral shape. If the spiral design is used, it is best constituted by means of a groove that is spirally arranged in the dielectric.

In the hitherto known sprinkling devices, it was basically possible to achieve a maximum spraying output of 200-600 g/min., when use was made of electrodes with sharp ring-shaped edges, middlepoints, or, in case of an indirectly ionizing charge, by means of an electrode arranged centrically in the sprinkler head, because as the volume of the material to be sprayed is increased, its relative charge naturally becomes smaller. The minimum charge volume which can still be used during spraying is 0.7 microcoulomb/g, related to the unit of material volume sprayed. This minimum charge volume is attained in hitherto known prior art spraying devices with the previously mentioned maximum sprinkling results. On the other hand, with the method and equipment according to the present invention, it is possible to improve the result considerably through the considerably greater length of the material path on which this material is charged. Thus, any desired large material volume can ge charged without the relative charge of that material becoming too small. This is of extraordinary importance in the design of highcapacity modern electrostatic spraying devices.

The invention can be used advantageously not only in manual spraying devices but also in all spraying and coating devices based on the principle of electrostatic charging.

BRIEF DESCRIPTION OF THE DRAWINGS Further details and advantages of the invention will emerge from the following description of the examples illustrated in the drawing:

FIG. 1 shows the basic principle of operation of the method according to the present invention;

FIG. 2 illustrates a longitudinal cross-section of one device according, to the present invention used in implementing this method;

FIG. 3 shows a longitudinal cross-section of another design for carrying out the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the basic principle of operation illustrated in FIG. 1, there is provided a condenser made up of two electrodes 1 and 4 which are spaced a short distance from each other opposite one another, and which have the shape of ribs or strips. An electrode 1 is connected via an insulated cable 3 with a high-voltage power source 2, whose other pole is grounded. The opposite electrode 4 is grounded and is coated with a dielectric material 5 facing toward electrode 1. Between dielectric 5 and electrode 1 is the passage or duct 8 for the material to be sprayed which is fed to this duct 8 through a pipe 7. At the exit end of this duct, an endpiece 6 is connected to the electrode 1. This endpiece does not have any current passing through it and is not excited, and, together with the opposite end of the dielectric 5, constitutes the exit end 9 or nozzle of the supply duct 8 for the spraying material.

The material to be sprayed, which is brought in through pipe 7, is charged between the two electrodes 1 and 4 by the strong force field which develops between the'se two electrodes so that when the charged material 8 reaches the end 9 of the supply duct, it crosses the spraying space 10 and is precipitated on the object to be sprayed, which has the same potential as the counter-electrode 6 and which can be grounded for safety reasons.

In the embodiment illustrated in FIG. 2, the spraying member as well as the charge member of the spraying device are illustrated schematically in a cross-section. In this embodiment, the two electrodes 1 and 4, which constitute a condenser in the supply duct for the material to be sprayed, are made in the form of pipes arranged concentrically inside each other, with the dielectric 5 located between these pipes.

Along the generated surface of dielectric 5 there is a spiral-shaped groove 8 which is connected with the material feed 7 and which represents the supply duct for the material to be sprayed, leading to'the spray head of the device. The external electrode 1 is connected, by means of an insulated cable 3, with a high voltage power source 2, whose other pole is grounded, while the internal electrode 4 is likewise grounded. The material supply or feed line is made of insulating material.

The end of electrode 1, which is opposite the material supply or feed line, joins a nozzle-shaped spray head 6 through which the material to be sprayed is conveyed. This material has previously been charged, while inside the spiral groove of dielectric 5, by means of the force field created in this groove between the two electrodes 1 and 4.

A portion of the length of the external electrode 1 is surrounded by a housing 12 which is made of insulating material and which, with electrode 1, forms an annular duct having an inlet 13 for compressed air. The end of the housing 12 adjacent the spray head 6 has an opening which leaves a ring-shaped crack or annular opening 14 around the spray head 6. Through this crack or opening 14 on the edge of the spray head 6, an air current, supplied from inlet duct 13, exits from the housing 12, whereby velocity of the spraying of the material out of the spraying head 6 is further increased.

The spraying material which is supplied under pressure through the supply or feed line 7 travels through the device, going toward the latters exit end 9, along a relatively long spiral path between the two electrodes 1 and 4 which are placed concentrically inside each other and between which there is a strong force field. This guarantees extraordinarily uniform and extremely strong charging of the material even prior to spraying, so that this charged material, after leaving the spraying device, can be precipitated or deposited in an even layer thickness on the object to be sprayed or coated.

If the pressure of the material flowing toward spraying head 6 is great enough and if the exit opening 9 in the spraying head 6 is correspondingly small, the mate rial is atomized as it comes out of the spraying head. If the material supplied to the spraying head has a relatively low pressure, atomization is accomplished with the compressed air which comes out of the housing 12 through the ring-shaped crack or opening 14 along the circumference of the spraying head.

In the manually operated atomization equipment shown in a cross-section in FIG. 3, the material, which is to be atomized and which is to be electrically charged, flows via the introduction line 17 and the intermediate duct 7 toward the supply duct 8. A valve 19, to be activated with a pushbutton or handle 20 is arranged between the introduction line 17 and the adjoining intermediate duct 7. The atomization air in this equipment unit moves via the supply or feed pipe 16 to pipe 13. Between these two pipes there is also arranged a valve 18 which can be operated by means of pushbutton or handle 20.

In operating the equipment, the pushbutton 20 is depressed to open the connections between ducts 7 and 17, respectively, and between pipes 13 and 16 respectively. As a result, the material to be sprayed, streaming into duct 17, reaches the influx duct 3 via intermediate duct 7. The influx duct 6 is surrounded by a dielectric 5 which is arranged inside a pipe-shaped elecn'ode 4 which is wounded via the handle 15 of the equipment. Within the supply duct 6 near the spray head 6, is positioned the second, cylindrically shaped electrode 1, which is connected via an'insulated cable 3 with a high voltage power source 2 whose other pole is grounded. Between this electrode 1 and the external electrode 4, a strong force field is developed in the supply duct 6 and through this force field the material to be sprayed, which flows in this duct toward sprayer head 6, is evenly charged. After electrostatic charging, this material, which reaches spray head 6, is, at the latters exit end 9, atomized either through the material pressure itself or by means of compressed air which is supplied via pipes 16 and 13 and adjoining annular shaped duct 21, which terminates into a ring-shaped exit crack or opening 14 which extends around the spray head 6. The annular shaped duct 21 and the exit opening 14 are formed by the cylindrical housing 12 which surrounds the external electrode 4 along a portion of its length. The electrostatically charged material, thus atomized, then streams via a spraying space or chamber 10 toward the object 11 to be sprayed, which is not shown in FIG. 3, which has the same potential as electrode 4, thus resulting in a uniformly and densily deposit of material.

Cylindrical housing 12 is attached to a grounded handle 15 which makes it possible to operate this equipment completely without any danger of electric shock.

As illustrated in FIGS. 1 and 3, electrode 1 can be made shorter than the counter-electrode 4 or can be spaced further from the exit end of the supply duct on the spray head than the counterelectrode because the conductor or semi-conductor, which is in contact with electrode 1, assures the desired charge along the supply duct 6 in combination with the longer counterelectrode. This shortening of the length of electrode 1 or increasing the spring from the exit end of the supply duct on the spray head results in even greater safety, especially in terms of the potential spark hazard.

What is claimed is:

1. A device for electrostatically charging and spray ing material on an object having an opposite electrical charge comprising a spraying head, a supply duct for supplying the material to be sprayed to the spraying head, an electrode connected to a high voltage source for obtaining an electrical charge from said high voltage source, the electrical charge of said electrode being opposite the charge of the object being sprayed, said electrode which is oppositely charged with respect to the object to be sprayed being positioned opposite a counter-electrode at a small spaced-apart interval therebetween, wherein one of said electrode or counter-electrode is covered with a dielectric, the dielectric facing toward the opposite electrode and wherein said electrode and counter-electrode form a condenser through which the sprinkling material is conducted, said condenser being positioned in front of the spraying head inside the supply duct for the material.

2. A device according to claim 1, wherein the electrode connected to the high-voltage source and which is oppositely charged by this source with respect to the object to be sprayed, is positioned a greater distance from the exit end of the supply duct on the sprinkler head than the counter-electrode of the condenser which is opposite it.

3. A device according to claim 1 wherein said electrode and counter-electrode of the condenser are comprised of pipes positioned concentrically, one inside the other, the supply duct for the material to be sprayed being formed by the space between said two pipes, said two pipes being further separated by a dielectric.

centrically inside each other.

5. A device according to claim 3, wherein the supply duct for the material to be sprayed is formed spirally between the two pipe electrodes arranged concentrically inside each other.

6. A device according to claim 5, wherein said supply duct for the material to be sprayed formed spirally between the two pipe electrodes arranged concentrically inside each other, consist of a groove which is spirally 4. A device according to claim 3, wherein the supply 10 formed in the dielectric between said two pipe elecduct for the material to be sprayed is formed cylindrically between the two pipe electrodes arranged controdes. 

1. A device for electrostatically charging and spraying material on an object having an opposite electrical charge comprising a spraying head, a supply duct for supplying the material to be sprayed to the spraying head, an electrode connected to a high voltage source for obtaining an electrical charge from said high voltage source, the electrical charge of said electrode being opposite the charge of the object being sprayed, said electrode which is oppositely charged with respect to the object to be sprayed being positioned opposite a counter-electrode at a small spaced-apart interval therebetween, wherein one of said electrode or counter-electrode is covered with a dielectric, the dielectric facing toward the opposite electrode and wherein said electrode and counter-electrode form a condenser through which the sprinkling material is conducted, said condenser being positioned in front of the spraying head inside the supply duct for the material.
 2. A device according to claim 1, wherein the electrode connected to the high-voltage source and which is oppositely charged by this source with respect to the object to be sprayed, is positioned a greater distance from the exit end of the supply duct on the sprinkler head than the counter-electrode of the condenser which is opposite it.
 3. A device according to claim 1 wherein said electrode and counter-electrode of the condenser are comprised of pipes positioned concentrically, one inside the other, the supply duct for the material to be sprayed being formed by the space between said two pipes, said two pipes being further separated by a dielectric.
 4. A device according to claim 3, wherein the supply duct for the material to be sprayed is formed cylindrically between the two pipe electrodes arranged concentrically inside each other.
 5. A device according to claim 3, wherein the supply duct for the material to be sprayed is formed spirally between the two pipe electrodes arranged concentrically inside each other.
 6. A device according to claim 5, wherein said supply duct for the material to be sprayed formed spirally between the two pipe electrodes arranged concentrically inside each other, consist of a groove which is spirally formed in the dielectric between said two pipe electrodes. 